The Quran

لَهُ مُعَقِّباتٌ مِن بَينِ يَدَيهِ وَمِن خَلفِهِ يَحفَظونَهُ مِن أَمرِ اللَّهِ ۗ إِنَّ اللَّهَ لا يُغَيِّرُ ما بِقَومٍ حَتّىٰ يُغَيِّروا ما بِأَنفُسِهِم ۗ وَإِذا أَرادَ اللَّهُ بِقَومٍ سوءًا فَلا مَرَدَّ لَهُ ۚ وَما لَهُم مِن دونِهِ مِن والٍ

(Saheeh International)

For him [i.e., each one] are successive [angels]¹ before and behind him who protect him by the decree of Allāh.² Indeed, Allāh will not change the condition of a people until they change what is in themselves. And when Allāh intends for a people ill,³ there is no repelling it. And there is not for them besides Him any patron.

Replacing each other by turn.
The phrase may also be rendered "…who guard him from [everything except] the decree of Allāh."
i.e., punishment or destruction because of their sins.